A CT scanner generally includes an x-ray tube supported by a rotating gantry, which is rotatably affixed to a stationary gantry. The x-ray tube emits radiation that traverses an examination region and a portion of an object or subject therein. A subject support positions the object or subject in the examination region for scanning. A radiation sensitive detector array is disposed across the examination region, opposite the x-ray tube, and includes a plurality of detector elements that detects radiation traversing the examination region and produces projection data indicative of the detected radiation. The projection data can be reconstructed to generate volumetric image data indicative of the portion of the object or subject in the examination region.
A conventional CT scanner typically includes integrating detectors, which typically include a scintillator array optically coupled to a photodiode array (e.g., of silicon photodiodes). Conventional CT scintillator are based, for example, on Gd2O2S (referred to as GOS), (LuTb)3Al5O12 (referred to as LuTbAG) or several forms of (Gd, Y, Ga)AG. The scintillator array includes scintillating material that absorbs x-rays that pass through the examination region and the portion of the object or subject and produces light in proportion to the total energy of the absorbed x-rays. The photodiode array absorbs the light produced by the scintillating material and converts the absorbed light into an electrical current in proportion to the light absorbed. The ideal detector produces a signal current in direct proportion to the x-ray intensity (i.e. the total energy of all photons in a reading) absorbed in the detector.
Such detectors have time dependent gains. Generally, the gain of a detector represents the transfer function from x-ray energy to an electric signal. Typically, the material selected for the scintillator of the detector array has a gain that is nearly constant over time. As such, an air calibration scan is performed to generate detector gain calibration data and subsequent air calibration scans can be made once every month or so to reflect changes in detector gain. Generally, an air calibration includes scanning with nothing located in the examination region so that the x-rays pass through the examination region without being attenuated and the gain of each detector is determined based on the output signal of the detectors. Any change in gain between calibrations typically may not be significant and has been neglected.
In some instances, the above-noted conventional CT detectors may not be the optimal choice or even appropriate, for example, for applications such as those involving spectral CT, photon counting, or protocols using very low detected x-ray flux. For such applications, detectors with higher gain (light output) or with spectral resolved properties is often the better choice. Scintillators with higher light output may be better suited since they offer a higher signal to noise ratio and reduced image artifacts in cases such as: 1) clinical protocols with low patient dose; 2) high speed scans (e.g., cardiac scans); 3) protocols with low tube voltage for high soft tissue contrast definition; 4) CT scanners having detector arrays with especially small pixels for high spatial resolution; 5) double-layer dual-energy CT for better material separation, and/or other advantages. Direct conversion materials may be the optimal choice for photon-counting spectral CT.
Several detector materials with higher light output (relative to GOS) are known. Examples of such materials include scintillator materials such as ZnSe (maximum (max.) ˜80,000 photons/mega-electron volt (ph/MeV)), Y2O2S (max. ˜63,000 ph/MeV), SrI2 (max. ˜90,000 ph/MeV), LaBr3 (max. ˜61,000 ph/MeV), Ba2CsI5 (max. ˜97,000 ph/MeV), etc., and direct conversion materials such as CdZnTe, CdTe, TlBr, GaAs, etc. For comparison, the conventional GOS scintillator can reach lower light output of max. ˜50,000 photons/MeV. Several light-element scintillators such as the aforementioned ZnSe may be well-suited for double-layer dual-energy CT detectors. From the aspect of light photodetectors, silicon photomultipliers (SiPM) or avalanche photodiodes (APD) can be used to achieve higher sensitivity.
Unfortunately, the above noted materials have unstable that the gains change more frequently over time, relative to the gain of the aforementioned conventional scintillators. As such, conventional approaches for calibrating for gain with a conventional scintillator, such as the example approach discussed above in which the gain is re-calibrated every month or so, are not well suited to be used to calibrate the gain of such materials. Therefore, there is an unresolved need for other approaches for calibrating detectors for gain.